Marine seismic exploration operations commonly include towing a seismic streamer behind a vessel. The seismic streamer includes data communications channels, power conductors, one or more strength members, and a number of sound-sensitive hydrophones. To maintain a very nearly neutral buoyancy, the streamer is commonly filled with a ballast fluid, such as kerosene or oil.
As the streamer is towed through the water during seismic operations, its primary function is to receive seismic signals at the plurality of hydrophones from subsurface geological structures, convert these signals to a voltage signal, and transmit these voltage signals to a central receiving station on board the vessel. The seismic signals are very often very weak, and can be masked by noise from a variety of sources. It is therefore imperative that these sources of noise be minimized so as not to interfere with the seismic signal of interest. This means that the signal to noise ratio of the sound receiving apparatus should be as high as possible.
Large diameter fluid-filled cables have achieved excellent signal to noise characteristics, but these cables are expensive, cumbersome, heavy, and not well suited to seismic operations in heavy weather at sea. Consequently, more recent fluid-filled cables have smaller diameters, at the cost of very fine signal quality. However, these smaller diameters cables are more robust, lighter, less expensive, easier to tow and operate, and have demonstrated adequate signal quality in most operating situations.
Even the smaller diameter streamer cables have their ballast fluid contained with a thin plastic jacket, typically 3-4 mm thick. This skin is susceptible to damage during normal streamer deployment and retrieval operations, and may also be easily damaged by objects in the water, by accidental contact with other streamers, and by a number of common hazards. Other internal components of the streamer cable are also susceptible to damage during normal streamer deployment and retrieval and from hazardous operating conditions. These factors, among others, have led to the developments today in solid-filled cables. Solid-filled cables are more robust and suffer less damage from normal operations and hazard conditions.
Solid-filled streamers include groups of hydrophones spaced apart along the length of the cable. Ideally, the hydrophones would be isolated from any noise in the cable, while positioned to receive the maximum amount of the seismic signal of interest. The hydrophones along the cable are commonly mounted within a hydrophone carrier, which is an integral portion of the cable.
Thus, there is a need for a hydrophone carrier in a solid-filled seismic cable which is robust, inexpensive, and easily accessible for repairs while the towing vessel is deployed at sea. The carrier should be as strong as the rest of the cable, during all phases of operation, including steady state steaming, heavy weather (which can induce longitudinal jerks in the cable) and deployment and retrieval operations in which the cable is reeled onto a winch. The carrier should also isolate the hydrophones from noise conducted along the cable, while exposing the hydrophones to the seismic signal without damping the signal.